With the advent of high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of three thousand copies per hour, the need for a document handler to feed documents to the copy platen of the machine in a rapid, dependable matter was recognized to enable full utilization of the reproduction machines potential copy output. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the originals and generate minimum machine shutdowns due to uncorrectable misfeeds or document multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur.
To provide a gentle yet positive feed, a vacuum feed belt assembly may be positioned beneath the stack of documents to be fed for acquiring the bottom document in the stack on the belts by vacuum and energizing the belts to pull the acquired document from under the stack into the document path of the document handler. To prevent misfeeds and multifeeds, an air knife may be positioned at the lead edge of the stack for injecting air between the documents in the stack to provide an air bearing between the document being fed and the remainder of the stack. This greatly reduces the force necessary to pull the bottom document from the stack and also minimizes the possibility of the adjacent document being pulled out from under the stack with the document being fed.
To simplify the feed system, a single blower may be used to provide both positive and negative air pressure for the air knife and vacuum feed belt assembly respectively.
However, if the sheet being fed is tightly drawn onto the vacuum feed belt assembly, the sheet will substantially block the flow of air to the inlet of the blower, thereby reducing the flow of air to the air knife below the level required to adequately lift the document stack and provide an air bearing between the sheet being fed and the remainder of the sheets in the stack.
Conversely, prior to a sheet being pulled down onto the vacuum feed belt assembly, air flow to the blower may be essentially unrestricted, resulting in excess air flow to the air knife. It is therefore the object of this invention to provide a sheet feeder having an automatic self compensating air flow control valve to provide the desired air flow through the system at all times.